1. Technical Field of the Invention
The present invention relates to the preparation of carboxylic acids by controlled oxidation of the corresponding alkanes, in the presence of heteropolyacid catalysts.
The present invention also relates to novel compositions comprising a heteropolyacid and a titanium dioxide support therefor, as well as to novel catalysts, such compositions comprising the active catalytic phase thereof.
As utilized herein, by the term "heteropolyacid" (or HPA) are intended both heteropolyacids and polyoxometallates.
2. Description of the Prior Art
The preparation of carboxylic acids, in particular acetic acid, via controlled oxidation of the corresponding alkane, such as ethane, is today widely used since it presents the advantage of employing a starting material which is very cost competitive vis-a-vis those employed in existing technologies.
Among the known processes of this type, representative are those employing, other than ethane and oxygen, catalysts based, at least, on the oxides of vanadium, phosphorus and rhenium. The selectivities in respect of acetic acid are on the order of 30% but they remain, however, less than those of the ethylene coproduced. In addition, such a process requires the introduction of large amounts of water and of an inert gas such as helium or nitrogen into the reactor.
Another known process entails reacting the ethane and oxygen in the presence of catalysts based on at least a mixture of the oxides of molybdenum, vanadium, niobium and antimony, or, alternatively, based on the oxides of molybdenum, vanadium, rhenium, antimony, niobium and calcium. The selectivity in respect of acetic acid when using such catalysts is enhanced relative to that attained via the above process. However, the reaction conditions are demanding. Indeed, it is necessary to carry out the oxidation under conditions combining an elevated temperature, namely, on the order of 300.degree. to 400.degree. C., and a high pressure, on the order of 20 to 30 bar. Moreover, the processes for the preparation of such catalysts are difficult to carry out.
It too is known to employ catalysts whose active phase comprises the oxides of vanadium, in the oxidation state (IV), of titanium and/or of phosphorus. When using these catalysts, their performances are of interest in terms of selectivity in respect of acetic acid. However, the starting material feedstream comprises more than 90% of diluent gas, which results in a very low productivity.